Prepare Mango Jam: Begin by heating 200g of frozen or fresh mangoes in a saucepan over medium heat until softened, about 5 minutes. Once softened, mash the mango and mix in 1 tablespoon of white sugar. Simmer for an additional 5-7 minutes until thickened to a jam-like consistency. Remove from heat and cool in the fridge.

Cook Sticky Rice: Soak 0.25 cup of sweet rice in boiling water for 20 minutes. After soaking, drain and steam the rice for about 30 minutes until underdone. Combine with 0.33 cup of heated coconut milk and 3 tablespoons of sweetened condensed milk. Reserve 1 tablespoon of this mixture for the cookie dough.

Make Cookie Dough: Cream together 113g of unsalted butter, 100g of light brown sugar, and 50g of white sugar until fluffy. Add the egg yolk, thick egg white, 0.5 teaspoon of vanilla extract, and 1 tablespoon of reserved coconut cream, mixing until smooth. Gradually incorporate 0.5 teaspoon of baking soda, 0.25 teaspoon of salt, and 180g of all-purpose flour until just combined.

Chill the Dough: Cover the dough with plastic wrap and chill in the refrigerator for 30 minutes. Preheat your oven to 350°F (175°C) and prepare your baking tray with parchment paper.

Assemble and Bake: Scoop cookie dough portions onto the tray, spacing them about 2 inches apart. Top with sticky rice and mango jam, swirling them together. If desired, sprinkle with black sesame seeds. Bake for 13-15 minutes until golden brown edges and set centers.

Cool and Serve: Allow the cookies to cool on the tray for 10 minutes, then transfer to a wire rack to cool completely. Enjoy these treats warm or at room temperature with your favorite beverage!